This project is a long-term commitment to achieving a detailed understanding of the molecular mechanisms involved in site-specific recombination. The Int-dependent integration/excision pathway of bacteriophage lambda has been chosen as a model system that is already highly developed for genetic as well as biochemical approaches. The formulation of models in other recombination pathways and the general problem of protein-nucleic acid interactions are two particular concerns emerging from these experiments. The specific aims are: 1) To determine, in very precise terms, how Int protein interacts with different att sites and with different binding sequences of a given att site at specified stages in the recombination reaction. 2) To determine how the host factor and Xis proteins affect the recombination reaction and to ascertain whether this is mediated through an effect on Int-DNA interaction. 3) To determine whether the crossover event is the result of cutting and religating DNA strands at a variable or fixed (where?) position within the core and whether the strands are cut in a flush or staggered configuration. 4) Recent results have led us to a model for a "recipient" and a "donor" att site. It is necessary to further test the implications of this model and determine precisely which structural features are responsible for the particular recombinational properties of each att site.